Implantable cardiac sensing and stimulating devices are generally used to manage a variety of heart arrhythmias and conduction system blockages. Heart arrhythmias, such as bradycardia and tachycardia, often prevent the heart from pumping an adequate amount of blood. When the body does not receive enough oxygen-carrying blood, symptoms such as fatigue, shortness of breath, dizziness, and unconsciousness may occur. Furthermore, conduction system blockages in the heart cause slow, asynchronous contractions that reduce the pumping efficiency and lower cardiac output. Implantable cardiac sensing and stimulating devices must be capable of detecting such arrhythmias and decreased pumping efficiency due to conduction system blockages, and the implantable device should respond to the detected arrhythmia or low pumping efficiency by providing therapeutic electrical stimulation.
Accurate measurement of cardiac activity is needed to deliver effective therapy by an implantable cardiac sensing and stimulating device. Many cardiac sensing and stimulating devices that detect and distinguish among cardiac arrhythmias monitor heart rate, which is usually accomplished by measuring cardiac electrical activity. Furthermore, the functions of the conduction system and synchronization of cardiac wall contractions are assessed by measuring and analyzing cardiac electrical activity. However, electrical activity is not a sufficiently accurate representation of the mechanical function of the heart. Thus, using only electrodes to sense cardiac mechanical activity can have some disadvantages in some circumstances.
Some implantable cardiac sensing and stimulating devices include implantable leads with built-in accelerometers to measure cardiac mechanical movement representative of cardiac contractile function. However, built-in accelerometers typically require a dedicated implantable lead, which tends to be bulky and hard to handle. Furthermore, these conventional leads with built-in accelerometers are too large to fit within a vein of a cardiac wall and require invasive installation procedures.
Thus, it is desirable to provide an improved sensing method and system for accurately detecting and monitoring cardiac mechanical activities. Further, it is desirable to provide an improved sensing method and system that has the ability to be implanted without a dedicated lead, such as within a preexisting implantable lead that may be positioned within a vein of a cardiac wall.